It is known that a photovoltaic semiconductor p-n junction can convert to electricity only that portion of the incident photon energy spectrum, typically solar radiation, which creates hole-electron pairs within a given semiconductor material. For example, in a silicon photovoltaic cell only that portion of the solar spectrum with energy slightly greater than or equal to the 1.1 electron volt band gap is efficiently converted into electricity. Photons of lesser energy are not absorbed at all. More energetic photons are absorbed and most of the energy lost in heating the cell. It is this heating and absorption process which can degrade the cell's energy conversion efficiency. To maximize the efficiency of a given photovoltaic cell, it is advantageous to convert as much of the available light as possible into an energy range to which such cell can optimally respond in the generation of electricity before the light strikes the cell's surface.
One technique for achieving such conversion takes advantage of the fact that light falling upon a luminescent agent is characteristically reradiated or emitted in a band of wavelengths of known energy content. Also, light absorbed by such an agent in one direction is reradiated isotropically. Such agents include, for example, pigments, metal oxides and organic dyes which are used in scintillation counters, lasers, and the like. For the purpose of this invention the term "luminescent agent" includes all types of luminescent agents exhibiting all species of luminescence, including, but not limited to, fluorescence and phosphorescence.
It has been shown that the dispersal of a luminescent agent within a sheet of glass or plastic, one of whose major surfaces is exposed to light, concentrates and focuses a flux of light of known energy toward one or more of the thin edge faces of the sheet. If a photovoltaic cell responsive to light at that energy level is optically coupled to such edge face, the energy conversion efficiency of the cell increases. In this invention a light transmissive member of such construction and properties is termed a "luminescent member" and a photovoltaic solar collector employing such a member is termed a "luminescent solar collector". A luminescent solar collector of this type is fully and completely disclosed in Optics, Vol. 15, No. 10, pages 2299-2300, dated October, 1976, the disclosure of which is incorporated herein by reference.
It is desirable once light enters the interior of the luminescent member that it be internally reflected therein until it finally reaches a photovoltaic cell and does not escape out of the luminescent member by refraction or otherwise before it reaches the photovoltaic cell. This invention improves the retention of light in the luminescent member of an operating luminescent solar collector when that collector is exposed to normal weather and other operating conditions.